This invention relates to a process for producing cyclopolydiorganosiloxanes. More specifically, this invention relates to a process which minimizes the cleavage of organic groups from silicon atoms while the cyclopolydiorganosiloxanes are being prepared and which improves the quality of the final product cyclopolydiorganosiloxanes.
To aid in the understanding of the instant invention, the following chemical definitions and notations are used herein:
"Polydiorganosiloxanes" means
a. Cyclopolydiorganosiloxanes of the general formula, EQU (R'R"SiO).sub.x, PA0 wherein x has a value of at least 3; PA0 b. Linear polydiorganosiloxanes of the general formula, EQU HO(R'R"SiO).sub.v H, PA0 wherein v has a value of at least 2; and PA0 c. A mixture of a. and b.; this mixture can include the product of hydrolysis of a diorganodihalosilane.
"Cyclopolydiorganosiloxanes" other than convertible cyclopolydiorganosiloxanes means the desired product which is a mixture that contains cyclopolydiorganosiloxanes having the general formulae, EQU (R'R"SiO).sub.w, (R'R"SiO).sub.y, and (R'R"SiO).sub.z,
wherein w has a value of 3, y has a value of 4, and z has a value of 5.
"Branching", as used in the instant invention, means trifunctionality or tetrafunctionality caused by the cleavage of organic groups from the polydiorganosiloxane structure. This cleavage is represented by the following reaction scheme: ##STR1## Branching is expressed as branched sites per million silicon atoms or parts per million branching.
Details on the chemical structure and notation will be set forth infra in this specification.
A significant quality issue with cyclopolydiorganosiloxanes is the branching content of the cyclopolydiorganosiloxanes which is caused by the cleavage of organic groups from the silicon atoms. Cleavage is particularly a problem with the vinyl and phenyl moieties on silicon. Siloxanes with a low level of branched sites are needed in the preparation of high-molecular weight diorganosiloxane polymers and their subsequent use in silicone elastomers.
The objectives of the instant invention are: (1) maximizing the yield of cyclopolydiorganosiloxanes; (2) minimizing the thermal or chemical cleavage of organic groups from silicon atoms; and (3) providing for a continuous reaction scheme for the preparation of cyclopolydiorganosiloxanes.
Most of the art that has been disclosed prior to the instant invention deals with the preparation of cyclopolydiorganosiloxanes by the thermal "cracking" or depolymerization of polydiorganosiloxanes in which the polydiorganosiloxanes are reacted with base catalysts to form the cyclopolydiorganosiloxanes, the cyclopolydiorganosiloxanes being continuously removed from the reaction zone by fractionation or other means of separation.
Hunter et al., J. Am. Chem. Soc., 68(1946), pp. 667-672, describe a method for preparing cyclic polydimethylsiloxanes of three to eight members from the product of the hydrolysis of dimethyldichlorosilane. A combination of distillation and depolymerization at temperatures up to 300.degree.-400.degree. C. using sodium hydroxide as a catalyst is disclosed. Individual cyclic polydimethylsiloxane species are isolated from the crude mixture of cyclic materials by fractional distillation.
Hyde in U.S. Pat. No. 2,438,478, issued March 23, 1948, and in U.S. Pat. No. 2,455,999, issued Dec. 14, 1948, discloses a process for recovering low-molecular weight cyclic polydimethylsiloxanes. The process is similar to that described by Hunter et al. above.
York in U.S. Pat. No. 2,816,124, issued Dec. 10, 1957, discloses a process for preparing hexaethylcyclotrisiloxane. This process utilizes limited amounts of alkaline-potassium catalyst such as hydroxide, carbonate, alcoholate, silanolate, and the like; heating high-molecular weight polydiorganosiloxanes under vacuum at 150.degree.-250.degree. C. with continuous removal of the cyclotrisiloxane.
Fletcher in U.S. Pat. No. 2,860,512, issued Nov. 11, 1958, discloses a method for preparing cyclic polydiorganosiloxanes by a "cracking" procedure in which polydiorganosiloxane materials are reacted with a hydroxide of Li, Na, K, or Cs in the presence of a high-boiling, inert solvent. The system is operated at high temperature and reduced pressure such as 225.degree.-230.degree. C. at atmospheric pressure and 130.degree.-150.degree. C. at 15-20 mm Hg. The solvent is used to overcome high viscosity or gellation in the reactor. This disclosure does not appear to deal with means to lessen the cleavage of organic groups.
Gordon in U.S. Pat. No. 2,884,432, issued April 28, 1959, discloses a process for preparing cyclic siloxanes in which triorganosiloxane materials are used to cause the contents of the cracker pot to remain fluid and to reduce the temperature of cracking for thermally less stable copolymers. Alkaline catalysts and temperatures of 150.degree.-160.degree. C. at 10-20 mm Hg are described. No mention is made of the impact of this invention on the quality of the final product.
Pierce and Holbrook in U.S. Pat. No. 2,979,519, issued April 11, 1961, disclose a process for the preparation of [(F.sub.3 CCH.sub.2 CH.sub.2)(CH.sub.3)SiO].sub.3 via "cracking" with fractionation. Catalysts utilized were KOH and LiOH. Temperatures of 200.degree. to 400.degree. C. were utilized. No mention is made of the problem of cleavage of organic groups.
Guinet and Puthet in U.S. Pat. No. 3,484,469, issued Dec. 19, 1969, disclose a process for preparing 1,3,5,7-tetramethyl-1,3,5,7-tetraphenylcyclosiloxane using an alkali metal carbonate as a catalyst in a "cracker" configuration. While improved yields are claimed, no mention is made of any improvement in final product quality by minimizing cleavage of the phenyl groups. Temperatures of 250.degree.-290.degree. C. were utilized.
Kuznetsova et al in U.S. Pat. No. 3,558,681, issued Jan. 26, 1971, disclose a process for the preparation of methylphenylcyclotri- and tetrasiloxanes by "cracking" using LiOH or lithium silanolate as the rearrangement catalyst. Temperatures of 250.degree.-360.degree. C. were used. Again, improved yields are claimed but no mention of any improvement in minimizing cleavage of phenyl groups, or any improvement in final product quality is made.
Macher in U.S. Pat. No. 3,607,898, issued Sept. 21, 1971, discloses a process for preparing cyclic symtetramethyltetravinyltetrasiloxane employing LiOH and a cocatalyst in a "cracking" configuration. Temperatures of 155.degree.-160.degree. C. at 20-30 mm Hg were utilized. While the problems of gellation in the reactor and low yields are lessened by this invention, there is no apparent reference to a measured improvement in lowering the tri- or tetrafunctionality of the final cyclopolyorganosiloxanes. A disadvantage of this disclosure compared to the instant invention is the need to use expensive co-catalysts such as alkylpolyethers and triphenylphosphine oxide.
Razzano in U.S. Pat. No. 3,846,464, issued Nov. 5, 1974, discloses a process for preparing symtetramethyltetravinyltetrasiloxane by cracking methylvinylpolysiloxanes with KOH and a high-boiling solvent. Temperatures in the cracking reactor were cited as 160.degree.-165.degree. C. at 20-30 mm Hg. As with the above cited patent by Fletcher, this disclosure deals with keeping the reaction mixture at a low viscosity and does not appear to address the problem of cleavage of organic groups.
Okamoto and Yanagisawa in U.S. Pat. No. 3,989,733, issued Nov. 2, 1976, disclose a combination "cracking" and rectification process in a column type reactor which uses as the column packing an alkaline catalyst in the form of pellets or an inert material upon which the alkaline catalyst is fused. Temperature and pressure conditions disclosed for the reaction zone were varied from 170.degree.-270.degree. C. and from 20 mm-760 mm Hg, respectively. While this disclosure improves on the semi-batch, semi-continuous schemes outlined above, the issue of improved quality due to reduced cleavage of organic groups does not appear to be addressed.
Finally, Bluestein in U.S. Pat. No. 4,111,973, issued Sept. 5, 1978, discloses a process for preparing a cyclotrisiloxane in a cracking reaction. This invention is only operable on fluoroalkyl siloxanes. Improved yields are claimed using higher aliphatic alcohols of 14 to 30 carbon atoms and temperatures above 150.degree. C. No mention is made of the quality of the final product.
Two disadvantages appear inherent in these prior art approaches to preparing cyclopolydiorganosiloxanes. First, in most cracker configurations, even though the cyclopolydiorganosiloxanes are continuously removed from the reaction zone, a portion of siloxane materials is continuously in contact with the basic catalyst at elevated temperatures during the course of an extended run. It has been found, as illustrated in the examples infra, that prolonged contact of the polydiorganosiloxanes with a base catalyst at reaction temperature significantly increase the organic group cleavage. Concentration of the base catalyst relative to the polydiorganosiloxanes also significantly impacts upon organic group cleavage. The two-step reactor configuration of the instant invention would have a total residence time of approximately 1 hour. Secondly, the use of vacuum in an apparent attempt to lower the cracking temperature poses potential processing problems when using hydroxy-endblocked polydiorganosiloxanes as feed materials. The generation of water, particularly in a continuous reaction configuration, could result in foaming which significantly complicates reactor pressure control. Further, this foaming due to the liberation of water vapor under reduced pressure causes carry-over of the basic catalyst into a fractionation or stripping device and causes polymerization of the cyclosiloxane product and subsequent gellation.
Several unexpected findings were discovered during the development of the instant invention. It was found that the reaction of hydroxy-endblocked polydiorganosiloxanes proceeded in two distinct steps: (1) a reaction to form water; and (2) rearrangement of polydiorganosiloxanes to form cyclopolydiorganosiloxanes. It was found that the reaction to form water proceeded much more rapidly than the rearrangement reaction (approximately an order of magnitude faster). A continuous reactor scheme was developed which allowed these reactions to proceed essentially separately.
A low-boiling solvent is used in the process for two purposes: (1) to shift the chemical equilibrium to yield a maximum cyclopolydiorganosiloxane content in the reactor effluent, a phenomenon known in the art (Carmichael et al., J. Phys. Chem., 71:7 (1967), pp. 2011-2015, discuss the effect of solvent dilution upon the equilibrium between dimethylsiloxane cyclics and linears); and (2) to form a two-phase azeotrope to remove water from the reactor system. The latter point is important, since it was found unexpectedly that the presence of water during the rearrangement reaction shifted the chemical equilibrium away from the product cyclopolydiorganosiloxanes in favor of linear polydiorganosiloxanes. It was found that in using a reaction mixture that was at least 70 weight percent solvent, the siloxane fraction of the mixture was approximately 70 weight percent or more of the desired cyclopolydiorganosiloxane (cyclic trimer, -tetramer, and -pentamer), if the water of reaction was removed before the rearrangement reaction. If the water of reaction was not removed prior to the rearrangement reaction, the cyclopolydiorganosiloxane content was lowered to approximately 60 weight percent. The impact of water on the equilibration of polydiorganosiloxanes is outlined in the examples.
The desired cyclopolydiorganosiloxanes can be separated from the solvent and higher-boiling siloxane materials by conventional means such as distillation. An unexpected finding in the recovery of the cyclopolydiorganosiloxanes was the discovery that branched species were distributed throughout the siloxane fraction. For purposes of this invention and in order to follow the reaction, identification and measurement of branched species was facilitated by a special gas chromatographic analysis technique. The findings herein indicate that the minimizing of cleavage during the reaction to form the cyclopolydiorganosiloxanes is critical in controlling the branching content of the final product. The apparent shortcomings of the cracking technique in this regard have been described above.